A photon counting apparatus such as a photon counting CT apparatus detects X-rays of a high dose on a photon basis and discriminates a substance that the X-rays have passed through. In the photon counting apparatus, a direct type detector such as a semiconductor detector having an excellent energy resolving power is used to discriminate a substance. As a readout circuitry used to read out a signal from the detector, a highly integrated element such as an ASIC is assumed to be used. Regardless of its high energy resolving power, the semiconductor detector has many problems such as a high cost and poor stability when used in the photon counting apparatus. On the other hand, an X-ray computed tomography apparatus uses an indirect type detector. The indirect type detector is formed by combining a phosphor such as a scintillator and a photodetector such as a photomultiplier. Such an indirect type detector is field-proven as a detector for an X-ray CT and features a low cost and high stability, as compared to the semiconductor detector. However, because of a low energy resolving power, it is very difficult to discriminate a substance.